In wellbores, annular barriers are used for different purposes, such as for providing a barrier for flowing between an inner and an outer tubular structure or between an inner tubular structure and the inner wall of the borehole. The annular barriers are mounted as part of the well tubular structure. An annular barrier has an inner wall surrounded by an annular expandable sleeve. The expandable sleeve is typically made of an elastomeric material, but may also be made of metal. The sleeve is fastened at its ends to the inner wall of the annular barrier.
In order to seal off a zone between an inner and an outer tubular structure or a well tubular structure and the borehole, a second annular barrier is used. The first annular barrier is expanded on one side of the zone to be sealed off, and the second annular barrier is expanded on the other side of that zone, and in this way, the zone is sealed off.
The pressure envelope of a well is governed by the burst rating of the tubular and the well hardware etc. used within the well construction. In some circumstances, the expandable sleeve of an annular barrier may be expanded by increasing the pressure within the well, which is the most cost efficient way of expanding the sleeve. The burst rating of a well defines the maximum pressure that can be applied to the well for expansion of the sleeve, and it is desirable to minimise the expansion pressure required for expanding the sleeve to minimise the exposure of the well to the expansion pressure.
When expanded, annular barriers may be subjected to a continuous pressure or a periodic high pressure from the outside, either in the form of hydraulic pressure within the well environment or in the form of formation pressure. In some circumstances, such pressure may cause the annular barrier to collapse, which may have severe consequences for the area which the barrier is to seal off as the sealing properties are lost due to the collapse. A similar problem may arise when the expandable sleeve is expanded by means of e.g. a pressurised fluid. If the fluid leaks from the sleeve, the back pressure may fade, and the sleeve itself may thus collapse.
The ability of the expanded sleeve of an annular barrier to withstand the collapse pressure is thus affected by many variables, such as strength of material, wall thickness, surface area exposed to the collapse pressure, temperature, well fluids, etc.
A collapse rating currently achievable of the expanded sleeve within certain well environments is insufficient for all well applications. Thus, it is desirable to increase the collapse rating to enable annular barriers to be used in all wells, specifically in wells that experience a high drawdown pressure during production and depletion. The collapse rating may be increased by increasing the wall thickness or the strength of the material; however, this would increase the expansion pressure, which as mentioned is not desirable.
It is thus desirable to provide a solution wherein the collapse rating of expanded sleeves is increased.